Several methods for treating organic wastes have been in use throughout the country for some time. Hereinafter the process of the present invention shall be explained in connection with the treatment of sewage sludge but it shall be understood that sewage sludge is but a portion of the types of materials which can be treated according to the present process, which materials could be defined by the more general term "organic solids" and it shall be understood that the same process described herein concerning sewage sludge shall apply with equal utility to other organic solids that may be initially too moist for efficient development of aerobic thermophilic organisms. Raw or undigested sewage sludge typically consists of approximately 6% solid material. Anaerobically digested sewage sludge is a mixture of approximately 4% solids and 96% water. Dewatering and concentration of sludge is generally accomplished either by centrifugation, filtration or evaporation, bringing it to a higher concentration of solids. To aid the thickening process, sludge is frequently conditioned by the addition of polymers, chemicals or the application of heat.
After digestion and concentration of sludge the next step is disposal, either to the atmosphere, the water or the land. Concentrated sludge can be disposed of to the atmosphere by incineration and the resultant ash disposed of in a landfill or the like. Concentrated sludge is also frequently disposed of to the water by dilution in a large volume or it can be disposed of to the land in a sanitary landfill mixed with other waste solids such as municipal refuse. Frequently, concentrated sludge is applied to the land as a humus-type fertilizer. When used as fertilizer, it is customary to air-dry the concentrated sludge before application to the land and air drying takes a long period of time. Alternately, heat may be applied to accelerate the drying process. It is obvious that all of these disposal methods are costly, some more so than others. It is equally apparent that many of these disposal methods may pose a threat to the environment, and in any case with the exception of agricultural use of digested concentrated sludge, disposal is almost entirely without benefit to the environment, and disposal of a sludge, including agricultural use, may be restricted by some governmental agencies.
The application of sewage sludge solids to the land as a liquid after anaerobic digestion, either with or without further drying has received considerable attention in recent years and appears an acceptable method of ultimate disposal. This method, however, has several principal disadvantages. First of all during cold weather months it is usually necessary to store much of this material and storage of this noxious, decomposing material is a nuisance to surrounding landowners. Considering the volumes encountered in some metropolitan areas, the problem of storage becomes a problem of space. The second major disadvantage is not only the increased cost of treatment to assure reduction of all pathogens, but also the cost of transporting a large volume and weight of material most of which is water to the disposal site. Another disadvantage of the other systems of disposal and treatment is that the digestion, thickening, conditioning, dewatering and air drying do not destroy all pathogens nor the germination potential of some seeds.